Shadows sweep Stonehenge in
southernEngland (right), where
centuries offarmers worshiped
the seasons that ruled their lives.
Built before 1500 B.C., this circle
of stones is thought by some to be
the world's oldest observatory.
Celestial movements on a
vasterscale-the spinningof neu
tron stars-inspirescientists at
Arecibo Observatoryin Puerto
Rico (below). Usinga 20-acre
receiving dish, they can measure
radio waves from such pulsars to
within a microsecond over sev
eralyears, a precisionunexcelled
even by atomic clocks over simi
lar periods.